Disentangling the Impact of Morphology on the Photoelectrochemical Performance of Nanostructured Anodic TiO₂: A Systematic Study of Anodization Potential at Constant Oxide Thickness

Understanding the photoelectrochemical water oxidation performance of highly textured semiconductors requires systematic studies capable of revealing relevant correlations among the synthetic methods used, the resulting morphologies, and their relationship to activity. Here, we carry out a systematic study on anodized TiO₂ to understand the impact of anodization potential on the transition from a nanoporous to a nanoring morphology and its influence on the semiconductor properties and photoassisted water oxidation. We demonstrate that the applied anodizing potential has a profound impact on the material properties, in particular, the morphology of the top layer as well as the size and shape of nanotubes. The anodized substrates produced at higher potentials possess superior photoelectrochemical properties, which are ensured by a hierarchical nanoporous-nanoring morphology, high donor density, and a more positive flat band potential. We further confirmed the generation of oxygen at the anodic TiO₂ under white light irradiation through scanning electrochemical microscopy (SECM). This work introduces a methodology to systematically synthesize and characterize the features of hybrid semiconductor materials toward materials with improved photoelectrochemical properties for renewable energy technologies.